JPH05247540A - High strength cold-rolled steel sheet for deep drawing and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a high strength cold-rolled steel sheet for deep drawing excellent in secondary working brittleness and plane anisotropy. CONSTITUTION:Steel contg., as essential components, <=0.004% C, <= l.0% Si, <=2.0% Mn, <=0.2% P, <=0.01% S, 0.05 to 0.1% Al, <=0.006% N, 0.01 to 0.1% Ti, 0.003 to 0.03% Nb and 0.0015 to 0.005% B and contg., as selective components, one or >= two kinds among Cu, Ni, Cr, Mo and Co is hot-rolled at 800 to 900 deg.C finishing temp., is coiled at <=650 deg.C, is cold-rolled, is thereafter subjected to continuous annealing within the temp. range of >=830 deg.C to the Ac3 transformation point or below and is subsequently subjected to skin pass rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet for deep drawing which is excellent in secondary work embrittlement resistance and has small in-plane anisotropy, and a method for producing the same.

[0002]

2. Description of the Related Art Recently, as cold rolled steel sheets for automobiles,
The demand for high-strength steel sheets is significantly increasing in order to reduce the weight of the vehicle body to reduce fuel consumption and to ensure the safety of passengers. Such high-strength cold-rolled steel sheets are used not only for the inner plates of automobiles, but also for the outer plates of hoods, trunks, fenders, etc., so they have excellent properties not only in press workability but also in cold crack resistance after working. Must be Conventionally, many technologies have been proposed for high-strength cold-rolled steel sheets having good workability. These are those in which P with less material deterioration is added as a strengthening component. However, in the method of performing box annealing of a steel sheet containing P, box annealing itself is far more efficient than continuous annealing in terms of efficiency and energy consumption. It is inferior to the above and cannot expect a high level of productivity. On the other hand, in the continuous annealing method which is excellent in productivity, it is necessary to add a large amount of strengthening component to the ultra low C steel. Japanese Patent Application Laid-Open No. 61-104031 discloses Mn and P as basic strengthening components, and
Japanese Patent No. 226 discloses a technique of using steel to which Si, Mn, and P are added as a basic strengthening component. However, since it contains the above-mentioned large amount of reinforcing components, deterioration of secondary work brittleness resistance is inevitable. Further, as disclosed in Japanese Patent Laid-Open No. 61-2467344, it is difficult for a high strength steel sheet to have both properties of high workability and excellent secondary work embrittlement resistance.

[0003]

DISCLOSURE OF THE INVENTION The present invention has sufficient characteristics in terms of material, especially in-plane, without deteriorating the deep drawability and without requiring fine control of the amount of added elements and manufacturing conditions. The present invention has been made to provide a high strength cold rolled steel sheet for deep drawing which has little anisotropy and is excellent in secondary processing brittleness resistance, and a method for producing the same.

[0004]

In order to ameliorate the above-mentioned drawbacks, the present invention has conducted various studies on chemical composition and manufacturing conditions, and as a result, as an essential component, by weight composition, C: 0.
004% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.2% or less, S: 0.01% or less, A
1: 0.05 to 0.1%, N: 0.006% or less, T
i: 0.01 to 0.1%, Nb: 0.003 to 0.03
%, B: 0.0015 to 0.005%, balance F
Steel having a chemical composition consisting of e and unavoidable impurities is used as a billet, hot-rolled at a finishing temperature in the range of 800 to 900 ° C., and wound at a coil winding temperature (hereinafter abbreviated as a winding temperature) of 650 ° C. or less, A high-strength cold-rolled steel sheet for deep drawing and a method for producing the same, which comprises cold rolling, continuous annealing in a range of 830 ° C. or higher and Ac ₃ transformation temperature or lower, and then skin pass rolling.

Further, by weight composition as a selective component, Cu:
0.05-2.00%, Ni: 0.05-2.00%,
Cr: 0.05 to 2.00%, Mo: 0.02 to 1.0
The high-strength cold-rolled steel sheet for deep drawing and the method for producing the same according to the preceding paragraph, characterized by containing one or more of 0% and Co: 0.05 to 1.00%.

[0006]

First, the reason for limiting the chemical composition and its action will be described. C: 0.004% or less The lower the C, the more advantageous the material. Further, if the amount of C is large, the amount of Ti necessary for fixing C is inevitably increased, and the amount of composite precipitates produced is increased, resulting in deterioration of the material. Especially when C is 0.
If it exceeds 004%, the quality of the material will start to deteriorate significantly, so the content is limited to 0.004% or less. Si: 1.0% or less Si is an effective element that gives proper strength to steel together with Mn, and is positively added according to the required strength, but Si is an element that promotes brittleness and chemical conversion. Since it is also an element that impedes processability, it is set to 1.0% or less. Mn: 2.0% or less Mn is an effective element that gives proper strength to steel similarly to Si, but is 2.0% or less in terms of cost. P: 0.2% or less P is an element effective for improving the strength, and is positively added when high tensile strength is required, but if it is contained in a large amount, the amount of grain boundary segregation increases and embrittlement occurs. That is, the secondary processing brittleness resistance is deteriorated, so the content is made 0.2% or less.

S: 0.01% or less If a large amount of S is also contained, grain boundary embrittlement is likely to occur, resulting in deterioration of secondary work embrittlement resistance. Therefore, it is desirable to reduce the amount as much as possible and the content is set to 0.01% or less. Al: 0.05 to 0.1% The steel according to the present invention is produced by coexistence of Ti and Nb (Ti, N
b) By forming composite precipitates presumed to be C and (Ti, Al) N, C and N are fixed and their harm is eliminated, and thus Al is useful as a precipitate forming element. Further, it is necessary to add 0.05% or more because it is advantageous for lowering the in-plane anisotropy represented by the Δr value described later. On the other hand, even if added in excess of 0.1%, the effect cannot be expected to increase, which is rather disadvantageous in terms of cost. Therefore, Al is made 0.05 to 0.1% in range. N: 0.006% or less It is desirable to reduce N as much as possible in order to improve the formability and the deep drawability like C. Further, the aging resistance is also deteriorated, so the content is made 0.006% or less.

Ti: 0.01 to 0.1% Only by reducing N and C, the formability and deep drawability equivalent to or better than those of the box annealed material cannot be obtained.
Formability and deep drawability are improved by adding carbonitride forming elements such as the above to completely fix the solid solution C and the solid solution N. Also, by precipitating and fixing N as TiN,
The added B is a solid solution B having an effect of improving the secondary working brittleness.
It can exist in the state of. That is, Ti is N,
In order to precipitate and fix C, it is effective to add C in an atomic equivalent amount or more to N. However, the addition amount is 0.0
If it is less than 1%, the effect of addition is not significant. On the other hand, since Ti cannot be expected to increase in effect even if added over 0.1%, Ti is set to 0.01 to 0.1%. Nb: 0.003 to 0.03% Nb is effective in improving the r value and elongation by the combined addition of Ti, and the effect becomes remarkable at 0.003% or more, but exceeds 0.03%. Addition causes a decrease in elongation. Therefore, Nb is made 0.003 to 0.03%. B: 0.0015 to 0.005% As described above, B is said to have a function of strengthening the crystal grain boundary like C, but excessive addition of B has a strong tendency to reduce the r value and the elongation. Therefore, the material is largely deteriorated, which is not preferable as a deep-drawing steel sheet. Therefore, B is added in the range of 0.0015 to 0.005% in which the above-mentioned effects are effectively exhibited.

Cu: 0.05 to 2.00% Ni: 0.05 to 2.00% Cr: 0.05 to 2.00% Mo: 0.02 to 1.00% Co: 0.05 to 1 0.000% All of these elements are effective for improving the strength, and are added positively when high tensile strength is required.
Even if added excessively, the effect cannot be expected to increase, and it will only increase the cost. Therefore, the upper limit is Cu, N
i and Cr are 2.00%, and Mo and Co are 1.00%. On the other hand, if the addition amount is too small, the effect cannot be remarkably exhibited, so the lower limit thereof is 0.05% for Cu, Ni, Cr and Co,
Mo is set to 0.02%. Further, in order to fully exert the effects of the present invention, Cu, Ni, Cr, Co and M
One or two or more of o are selectively added.

Next, the reasons for limiting the manufacturing conditions of the present invention and the operation thereof will be described. Finishing temperature of hot rolling: 800 to 900 ° C When the finishing temperature of hot rolling is less than 800 ° C, r value due to residual strain and decrease in elongation are caused, while when it exceeds 900 ° C, r value due to coarsening of crystal grains. Bring about a decline. Therefore, the finishing temperature of hot rolling is limited to the range of 800 to 900 ° C. Winding temperature: 650 ° C. or lower Conventionally, the winding temperature is generally 600 to 800 ° C., because the size of TiC precipitates is larger, and high temperature winding that provides a high r value and elongation is useful. It was target. On the other hand, at lower winding temperatures, TiC, (Ti, A
l) N nuclei are less likely to be generated, and the precipitation rate is slow, so that precipitation of precipitates is not completed, so that precipitation fixation is not sufficiently performed, resulting in reduction of r value and elongation. The inventors have conducted various experiments and studies on the relationship between the winding temperature and the material properties, and as a result, have a completely new finding that they exhibit good secondary work embrittlement resistance and low in-plane anisotropy even at low temperature winding. I was able to get That is, FIG.
As shown in FIG. 2 and FIG. 2, a high strength cold rolled steel sheet for deep drawing showing low in-plane anisotropy (Δr) and excellent secondary work embrittlement resistance (shown in brittleness transition temperature ° C) even at low temperature winding. It was discovered that it can be manufactured.

In the steel of the chemical composition according to the present invention, by adding Al, the (Ti, N
b) C, (Ti, Al) N or other complex precipitates are precipitated, and the precipitation is promoted by low-temperature winding.
It is presumed that the in-plane anisotropy (Δr) was lowered because the precipitation and fixation of No. 2 occurred sufficiently and the crystal grains after hot rolling were also refined at the same time. The secondary work embrittlement resistance was improved because the formation of such precipitates promotes the segregation of B to the grain boundaries, reduces S as much as possible, and further reduces the grain size by hot rolling low temperature winding. Is presumed to have been miniaturized. As a result of such experiments and examinations, the winding temperature at which the in-plane anisotropy is small and the secondary working brittleness resistance is excellent is limited to 650 ° C or lower. In the actual operation, if you take into consideration the required cooling time, cooling capacity, coiled coil shape, etc., 3
00 ° C or higher is desirable. The test steels in FIGS. 1 and 2 had C = 0.003%, Si = 0.2%, and Mn = 0.3.
%, P = 0.07%, S = 0.006%, Al = 0.0
6%, N = 0.003%, Ti = 0.03%, Nb =
It was manufactured under the following manufacturing conditions using a steel having a chemical composition within the range of the present invention of 0.005% and B = 0.004%. Finishing temperature of hot rolling: 850 ° C. Winding temperature: 300 to 850 ° C. Continuous annealing temperature and time: 860 ° C. × 20 s Skin pass rolling: 1%

Continuous annealing temperature: 830 ° C. or higher and Ac ₃ transformation temperature or lower It is considered that the characteristics of the material are determined by the conditions during hot rolling, and the annealing temperature during continuous annealing has not been particularly discussed in the past. However, in JP-A-62-205231 and JP-A-58-19442, the recrystallization temperature or higher, Ac ₃
Although it is described as being below the transformation temperature, it is actually not specified. However, as a result of the inventors performing detailed experiments and studies on the annealing temperature, as shown in FIGS. 3 and 4, in-plane anisotropy (Δr), secondary work embrittlement resistance (brittleness transition temperature ° C. ) Was found to be greatly affected. This is because the precipitation at the grain boundaries of B is insufficient in annealing at 830 ° C. or lower, so that the secondary work embrittlement resistance cannot be improved. Further, regarding the in-plane anisotropy, It is estimated that the in-plane anisotropy (Δr) did not decrease because the orientation immediately after the crystal was influenced by the orientation formed during hot rolling.

On the other hand, above the Ac ₃ transformation temperature, deterioration of secondary work embrittlement resistance due to grain coarsening and increase of in-plane anisotropy (Δr) due to transformation occur. From these results, the continuous annealing temperature is limited to a range of 830 ° C. or higher and Ac ₃ transformation temperature (about 930 ° C.) or lower, which provides good secondary work embrittlement resistance and low in-plane anisotropy. The test steels in FIGS. 3 and 4 have C = 0.004%, Si = 0.4%, Mn = 0.3%,
P = 0.07%, S = 0.006%, Al = 0.06
%, N = 0.003%, Ti = 0.025%, Nb =
Using the steel having the chemical composition of the present invention of 0.01% and B = 0.0025%, the steel was manufactured under the following manufacturing conditions. Finishing temperature of hot rolling: 880 ° C. Winding temperature: 600 ° C. Continuous annealing temperature and time: 700 to 950 ° C. × 20 s Skin pass rolling: 1%

[0014]

[Example] Using a laboratory vacuum melting furnace, steel having the chemical composition shown in Table 1 was melted and hot-rolled under the conditions shown in Table 2,
The thickness was 3.5 mm. After hot rolling, coil winding treatment was performed under the conditions shown in Table 2, and cold rolling was performed to a thickness of 1.2 mm. Next, continuous annealing was performed under the conditions shown in Table 2, and then 1% skin pass rolling was performed. Table 2 shows the materials of the steel plates thus obtained and the results of the secondary work cracking test. In addition,
The secondary work crack test is a conical cup test defined in JIS (Z-2249), and the test piece diameter is 50.
Die hole diameter = 24.4m after punching out test pieces with mm
m, punch diameter = 20.64 mm, and the maximum temperature at which brittle cracking occurs when a crushing test is performed after cylindrical molding. In the test steel numbers A, B, C, D, E, F, G, H, and I, which satisfy the chemical composition and manufacturing conditions of the present invention, the test numbers 1 to 9 are all TS ≧ 39.9 kgf / m.
Maximum temperature at which brittle cracks occur (brittle transition temperature) with excellent materials with m ² , elongation ≧ 25% and r value ≧ 1.8
≦ −100 ° C., Δr ≦ 0.22, practically no brittle cracking occurs, and the in-plane anisotropy is very small. On the other hand, test numbers 11 to 12 according to the test steel symbols A and B whose chemical composition is suitable but whose manufacturing conditions are outside the scope of the present invention
Then, the temperature at which brittle cracks occur is high, and Δr ≧ 0.75
And the in-plane anisotropy is also large.

Further, in Test No. 13 using the symbol K of the test steel of the comparative example, the secondary work brittleness resistance is deteriorated and the brittleness transition temperature is also high because the high S and B are not added. Further, since the amount of addition of Ti is too small, solid solutions C, N, and S remain, and elongation and r value are reduced. Further, in the test number 14 using the test steel symbol L of the comparative example, since Si is too high, the secondary work embrittlement resistance deteriorates, and due to the low Al, the in-plane anisotropy (Δ
This results in an increase in r), an elongation due to no addition of Ti, and a decrease in r value. Further, in Test No. 15 based on the test steel symbol M of the comparative example, high P, high S, and low B deteriorate secondary work embrittlement resistance, and low Al and Nb do not add elongation and decrease in r value. Is invited. Due to these causes, even though the manufacturing conditions are within the range of the present invention, the brittle cracking maximum temperature (brittle transition temperature) ≧ −60 ° C. and Δr ≧ 0.65, and good characteristics are Not obtained. As described above in detail, only when both the chemical composition and the manufacturing conditions limited by the present invention are satisfied, the secondary processing brittleness resistance is excellent and the in-plane anisotropy is high. It is possible to manufacture a high-strength cold-rolled steel sheet.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

EFFECTS OF THE INVENTION Conventionally, the addition of B adversely affects the quality of the material.
Subsequent processing brittleness was improved. On the other hand, according to the present invention, by adding Al and B and further limiting the coiling temperature, the continuous annealing temperature, etc., the secondary working brittleness resistance is excellent and the in-plane anisotropy is small and the deep drawing height is high. A strong cold-rolled steel sheet can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a winding temperature (° C.) and an in-plane anisotropy (Δr).

FIG. 2 is a graph showing a relationship between a winding temperature (° C.) and a brittle transition temperature (° C.).

FIG. 3 is a graph showing the relationship between continuous annealing temperature (° C.) and in-plane anisotropy (Δr).

FIG. 4 is a graph showing the relationship between continuous annealing temperature (° C.) and brittle transition temperature (° C.).

Claims (2)

[Claims] 1. As essential components, by weight composition, C: 0.004% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.2% or less, S: 0.01 % Or less, Al: 0.05 to 0.1%, N: 0.006% or less, Ti: 0.01 to 0.1%, Nb: 0.003 to 0.03%, B: 0.0015 to Steel having a chemical composition containing 0.005% of the balance Fe and unavoidable impurities is used as a slab, hot-rolled at a finishing temperature of 800 to 900 ° C., and wound at a coil winding temperature of 650 ° C. or less. ,
A high-strength cold-rolled steel sheet for deep drawing, which comprises cold rolling, continuous annealing in the range of 830 ° C. or higher and Ac ₃ transformation temperature or lower, and then skin pass rolling, and a method for producing the same. 2. As a selective component, by weight composition, Cu: 0.05 to 2.00%, Ni: 0.05 to 2.00%, Cr: 0.05 to 2.00%, Mo: 0.02. .About.1.00%, Co: 0.05 to 1.00%, and one or more types thereof are included.
A high-strength cold-rolled steel sheet for deep drawing and a method for producing the same.